how to make ocean with depth
Here a possible solution (will only work in cycles): 


Instead of using a plane, turn your ocean into a volume that
stretches from the surface to the bottom of the sea. 
Give it a Principled BSDF with perfectly white Base Color, Roughness set to about 0.0 and Transmission somewhere at 1.0 (all values can of course be tweaked at the end)
Beneath the surface-tab (where you just set your Principled BSDF) is a volume-tab. Open it, and choose for Volume the Volume Absorption shader. Set the Color to something ocean-ish and tweak the Density until you're satisfied. 
Render and profit!


A little demonstration:



The red Suzanne is behind the ico-sphere, the spheres material (visible on the right) is something like you'd want your ocean to be. Here's a render: 



The lighting is off but you get the idea. Not only can you see the Suzanne through the sphere, things like refraction, etc. are handled for you automatically.